1. Technical Field
The present invention relates to scanners and bar code readers. More particularly it relates to a laser scanner having 2-D resonant electrostatic micro-mirror that enables projection or acquisition of a discrete image. The present invention also relates to a method of improving aiming visibility, reducing tilt dependence and improving read range of solid state 1D/2D stacked bar code readers.
2. Description of the Related Art
Two primary types of prior art scanners and bar code readers include solid state readers and laser scanners. Solid state bar code readers include CCD (charge coupled device) and CMOS (complimentary metal oxide semiconductor) readers. CCD readers use a row or array of photocells. A line or rectangular areas of a bar code is illuminated by a light source, typically LEDs built into the scanner. A 1D bar code is typically a pattern of light and dark lines of varying width. A 1D bar code does not vary vertically. Information is stored along only 1-dimension, the width, of a 1D bar code. A 1D bar code can be read with a reader having a line of LEDs illuminating a line across the width of the bar code.
There are many types of 2D bar codes. A 2D bar code may use stack bar code symbology, a multi-row code or a matrix symbology. A 2D bar code stores information along the width and height of the bar code. The pattern varies both horizontally and vertically. To read a 2D bar code, the scanner needs to have an array of LEDs illuminating the rectangular area of the 2D bar code.
The photosensors 20 in the CCD scanners reads the bar code illuminated by the light source. CMOS readers also capture light on a grid of small photosensors 20. Prior art solid state bar code scanners rely on a lighting path 22 encompassing the image plane and the 3D volume of view 26.
Because the lighting path must encompass the 3D volume of view, prior art solid state bar code scanners can only “read” what is illuminated. Thus, one disadvantage is that these readers cannot be used with bar codes that are wider or larger than the lighting path.
A CCD scanner does not have to be in direct contact with the surface of the bar code; however, its depth of focus is limited. Thus, another disadvantage is that these prior art scanners cannot be used for far field scanning. CCD scanners suffer from low brightness when used at a far field. Further, there is low photometric coupling efficiency. In other words a portion of the light is not captured by the photosensors 20.
Yet another disadvantage with prior art readers, is that they are tilt dependent when reading high-resolution 1D bar codes and thin row 2D stacked bar codes. In other words, the scanner works significantly better when the light is substantially perpendicular to the bar code. The scanning ability is diminished as the light hits the bar codes from a greater or lesser angle such as occurs when the scanner is tilted.
Laser scanners use a moving light source to illuminate the bar code and a photocell receives the light reflected from the bar code. Oscillating mirror(s) sweep the beam across the bar code. The beam can be swept horizontally to read 1D bar codes or the beam can sweep horizontally and vertically in a raster pattern to read 2D bar codes. Because the light emitted from a laser diverges very little as it travels, laser scanners can be used at far field.
In raster scanning, two mirrors scanning in orthogonal directions or one mirror scanning in two dimensions, and a modulated light source generates a 2-D image on a screen.
In other laser scanners, an optical imaging of a line of pixels or matrix of pixels is swept over the scene by a scanning mirror to image the scene as a whole. Pixels can be diffractive or reflective elements.
In digital light processing (DLP) displays, reflective LCD displays, a matrix of micro-mirrors is imaged as a whole onto the projection display to get a 2 dimensional image. The 2-D array of mirrors are less sensitive to the mirror surface curvature and scan with more uniformity than scanners using a 1-D array or raster-scanned displays. Moreover, the electronic throughput is lower and spatial resolution is better.
In grating light valve displays, a line of pixel is made of a 1-dimensional array of light modulators whose the image is swept over a screen by a scanning mirror to get a 2-dimensional image.
A disadvantage of 1-D or 2-D pixel array displays is that they require successful fabrication of many elements while raster-scanned display requires only one element.
Raster-scanned displays can be made smaller and at less cost than 2-D and 1-D scanned displays, making them more appealing for portable display applications.
Prior art, full electrostatic driven single mirror arrangements with electrodes underneath the mirror provide some degree of linearization. However, these scanners suffer from cross-talk, high voltage driving, and are difficult to package at the wafer level.
Prior art, 2-D gimbals suspended mirror with electrodes in the chip plane have simplified packaging with low pressure cavity at the wafer level, high frequency operation at low power driving, and allow independent excitation of both axis and the doesn't suffer from cross talk. However, these are difficult to manufacture and are fragile. The mirrors have low driving torques. As a result, both axes must be actuated in resonance so the oscillations are nonlinear. High Q factor and manufacturing tolerances make it virtually impossible to drive synchronously both axes at predefined frequencies. Moreover, low oscillation frequency value leads to fragile device because of long suspension hinges.
Existing miniaturized optical scanning for projection displays are expensive and power hungry. The current emerging display technologies target large format displays and do not meet the weight and power requirements of mobile applications. There is a need for durable, lightweight, low-power, inexpensive video displays that can be readily manufactured.